This invention relates to an optical visual apparatus, and more particularly to a half mirror for use with a small size light-weighted optical visual apparatus which can be mounted on the head of a user.
Head-mounted display units for virtual reality, for medical care, for military use, for a computer or for individual enjoyment with a large screen are known as optical visual apparatus.
Further, in recent years, a head-mounted display unit with which an image from a two-dimensional display element formed form a CRT, an LCD or a like element is magnified as a virtual image to be observed draws attention and is developed.
In the head-mounted display unit just described, the system for magnifying an image form the two-dimensional display element as a virtual image to be observed is such as shown in FIG. 24. Referring to FIG. 24, the system includes a two-dimensional display element 10 for displaying an image, and a half mirror 20 in the form of a parallel plate of a predetermined thickness for branching an optical path. The half mirror 20 has a half-coated surface 21 on the side thereof into which light is introduced. The system further includes a concave mirror 22 for magnifying the image as a virtual image to be observed.
When a user of the two-dimensional display element having the construction described above tries to observe an image from the two-dimensional display element as a magnified virtual image, light 23 from the half mirror 20 illuminated with a light source unit not shown is branched by the half mirror 20. Light 24 reflected thereupon from the half-coated surface 21 toward the concave mirror 22 side is subsequently reflected by the concave mirror 22 and forms an optical path of regular light 26 which enters an eye-ball 25.
On the other hand, light 27 coming into the half mirror 20 refractively through the half-coated surface 21 is branched into light 28 which goes out through the opposite surface of the half mirror 20 and light 29 which is reflected by the opposite surface of the half mirror 20 and then goes out refractively through the half-coated surface 21 toward the concave mirror 22. The latter light 29 forms an optical path of ghost light 30 which is reflected from the concave mirror 22 and enters the eyeball 25.
In this manner, the half mirror 20 formed from a parallel plate of a predetermined thickness never fails to have an optical path of the light 29 which comes into and goes out from the half mirror 20, that is, the ghost light 30, unless it can totally reflect the incoming light the half-coated surface 21 thereof.
However, the optical visual apparatus which includes the two-dimensional display element 10 for displaying an image, the half mirror 20 for branching an optical path and the concave mirror 22 for magnifying the image as a virtual image to be observed has several problems to be solved. In particular, in the optical visual apparatus, when light from the two-dimensional display element 10 illuminated with the light source unit is branched by the half mirror 20, light 24 reflected from the half-coated surface 21 is magnified by the concave mirror 22 and observed as regular light 26. Simultaneously, however, light having passed through the half-coated surface 21 comes to the opposite surface of the half mirror 20, and most light 28 of it passes through the opposite surface, but part of the light is reflected from the opposite surface. The reflected light 29 passes refractively through the half-coated surface 21 and is then reflected by the concave mirror 22 to make ghost light 30 which enters the eye-ball 25 and is observed as a magnified virtual image.
When an image formed from the regular light 26 and another image formed from the ghost light 30 are observed simultaneously, the image composed of the two images becomes a double image as seen in FIG. 26 and the original image may not sometimes be recognized because such a double image is indistinct. Further, observation of such a double image for a long time tires the eyes.
On the other hand, if a beam splitter or a prism is used in order to eliminate the ghost light 30, then this gives rise to such problems that the apparatus is degraded in balance or in mounting feeling to the user or, when the apparatus is used for a long time, the apparatus is displaced on the head and the user cannot observe the image well.
Further, where external light is to be observed as see-through light, if a beam splitter or a prism is used, then the light is spectrally diffracted due to an angle characteristic of a coating on the beam splitter or by the prism. Consequently, the external image is colored and is so hard to observe that the original image cannot be observed regularly. Further, observation of such an image for a long time tires the eyes.